Modern anti-theft devices for motor vehicles use electronic immobilizers which employ transponder technology. With such electronic immobilizers there is data communication between a transceiver arranged in the motor vehicle and a transponder arranged for example in a key or a key fob of the vehicle user. Before the motor vehicle is started up coded data is first exchanged which ensures that only an authorized person, for example the owner of the motor vehicle, can start it.
Such an anti-theft system for use in motor vehicles in which data communication is established using a magnetic coupling between a transceiver and a transponder is described in Patent DE 195 46 171 C1. Data is transferred for example between transceiver and transponder by an alternating field created magnetically by the transceiver being switched on and off according to the data code to be transferred. For these purposes the transceiver features an oscillating circuit which is excited via a driver circuit the inductance of this oscillating circuit is coupled magnetically with a corresponding inductance of the transponder oscillating circuit. Data communication between transceiver and transponder is bidirectional and uses the relevant oscillating circuits of transceiver and transponder. For this data communication an interrogation signal is first sent from the stationary transceiver to the transponder. The energy transmitted in this case can be stored in an energy store and when sufficient energy is present in the transponder the response code signal is triggered.
After sending the transceiver will be switched off by connecting the oscillating circuit to a reference potential. The oscillating circuit then oscillates in accordance with its quality. This decay process lasts a relatively long time (in the area of around 20 oscillations). The transponder now oscillates at its own resonant frequency and thus independently of the send frequency of the transceiver. After this phase independent of the data code to be transmitted, in which the transceiver is switched off, the transmitter unit of the transceiver is switched on again. To suppress damaging interference between the send signal of the transceiver and the inherent oscillations of the transponder the transceiver must be switched back on again in the correct phase with the inherent resonant frequency of the transponder. For this to be done it is necessary for the transceiver to be able to receive this signal frequency of the transponder before the driver circuit of the transceiver is switched back on.
These received signals are however generally signals with a very small amplitude. Furthermore the OFF phases in which the driver circuit of the transceiver is switched off cannot be changed at random according to the transmission rate. It must therefore be ensured that after the signal amplitude is sent a residual oscillation to be found in the transceiver is decayed to negligibly small values during these OFF phases of the driver circuit of the transceiver.
The problematic aspect of this is that the time taken for the oscillating circuit to decay is often too long and the transponder for its part is already ready in this period to send coded data back to the transceiver. A further problem arises from the fact that after the transceiver or its driver circuit is switched on, aperiodic oscillations can occur even long after decay process because of charge still stored in the capacitor of the transceiver oscillating circuit. This problem is described in greater detail using FIG. 3.
For data transmission of an interrogation code signal the transceiver oscillating circuit is excited with a square wave voltage UT. Depending on the layout of the oscillating circuit, an oscillating current I with amplitude I′ is established. After the interrogation code has been sent by the transceiver, the driver circuit is switched of at point TAUS. To do this the corresponding switch is initially opened. The driver circuit is now at high resistance and is in the tristate condition. The oscillating circuit current thus flows over corresponding free-running diodes switched in parallel with the switches. This leads to a phase reversal of the driver voltage UT. This contraphase voltage very quickly de-excites the oscillating circuit, which causes the oscillating circuit current to assume the value of 0 amperes after very few periods. However a potential remains at the output of the driver circuit which corresponds to the residual voltage UC over the capacitor of the oscillating circuit. If the oscillating circuit is now to be set to receive, the reference potential side controllable switch of the bridge circuit is switched on so that the output of the bridge circuit is switched to ground. As a result of the residual voltage UC via the capacitor this leads to a new transient oscillation of current I. These transient oscillations of the current I lead to interferences with the signal sent by the transponder and received by the transceiver oscillating circuit. Since these received signals are typically transmitted at a low power and thereby also exhibit a lower amplitude, as a result of the interference with the periodically decaying residual signal there are malfunctions in data communication.
This problem can be worked around by selecting the times between receiving and transmitting to be large enough, which however leads to the maximum allowable data transmission rate between transceiver and transponder being limited. This is however frequently not desirable since it would mean that data communication between transceiver and transponder would take a very long time.
The above problem can further be worked around by a reduction of the maximum permissible oscillating circuit quality of the transceiver. Although this allows data to be transmitted more quickly, the reduction of the oscillating circuit quality goes hand-in-hand with other, secondary problems which one attempts to reduce by other measures. For example the relevant oscillating circuits as well as the activation circuits would have to be dimensioned very much larger, which on the one hand would lead directly to a greater energy consumption. In addition this would be a disadvantage in relation to costs.